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Superhydrophobic and Omnidirectional Antireflective Surfaces from Nanostructured Ormosil Colloids Adem Yildirim,†,‡ Tural Khudiyev,†,‡ Bihter Daglar,†,‡ Hulya Budunoglu,†,‡ Ali K. Okyay,†,‡,§ and Mehmet Bayindir†,‡,∥,* †
UNAM-National Nanotechnology Research Center, ‡Institute of Materials Science and Nanotechnology, §Department of Electrical and Electronics Engineering, and ∥Department of Physics, Bilkent University, 06800 Ankara, Turkey S Supporting Information *
ABSTRACT: A large-area superhydrophobic and omnidirectional antireflective nanostructured organically modified silica coating has been designed and prepared. The coating mimics the self-cleaning property of superhydrophobic lotus leaves and omnidirectional broad band antireflectivity of moth compound eyes, simultaneously. Water contact and sliding angles of the coating are around 160° and 10°, respectively. Coating improves the transmittance of the glass substrate around 4%, when coated on a single side of a glass, in visible and near-infrared region at normal incidence angles. At oblique incidence angles (up to 60°) improvement in transmission reaches to around 8%. In addition, coatings are mechanically stable against impact of water droplets from considerable heights. We believe that our inexpensive and durable multifunctional coatings are suitable for stepping out of the laboratory to practical outdoor applications. KEYWORDS: superhydrophobic, self-cleaning, organo-modified silica, antireflection, omnidirectional, functional surfaces
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INTRODUCTION Several biological organisms have been synthesizing functional nano- and microstructures in order to obtain desired surface property.1,2 Well known examples of these functional surfaces include self-cleaning surface of lotus leaves3 and antireflection structures on moth compound eyes.4 Lotus leaf surface comprises of micro-/nano-hierarchical structures, and the tops of these structures are coated with a thin hydrophobic wax layer. Such a hierarchically rough and low energy surface provides stable superhydrophobicity with high water contact angle, small hysteresis, and easy rolling of water droplets away from the surface. This type of superhydrophobicity can be explained according to the Cassie−Baxter model in which water droplets rest on a surface composed of air trapped in the microgrooves of a rough surface and tops of these microstructures.5,6 To obtain such a composite interface between water and the surface, it must be roughened conveniently and coated with low surface free energy materials.7,8 An ordered hexagonal array of subwavelength nipples on moth compound eyes, on the other hand, results in antireflection over a broad range of wavelengths by eliminating the destructive interference between air−array and array−surface interfaces.4 Inspired from such biological examples, many successful approaches have been developed to prepare functional surfaces with self-cleaning or antireflection property, using lithographic methods,9−13 deposition of micro-/nanoparticles,14−17 phase separation in polymers,18−20 layer by layer deposition methods,21−23 and sol−gel methods.24−28 © 2013 American Chemical Society
On the other hand, an optically transparent surface (e.g., glass, quartz, and PMMA) that exhibits both self-cleaning and antireflection features would be very beneficial in many applications such as solar cells, LEDs, and optical lenses since such highly transparent and water repellent surfaces can improve the device performance by eliminating the reflection losses and also offer low cost maintenance due to the selfcleaning property.29 However, the challenge in preparing such multifunctional surfaces is to balance the high surface roughness requirement of superhydrophobic coatings with the low surface roughness requirement of antireflection coatings.30 Therefore, to prepare antireflective and superhydrophobic surfaces, roughness must be optimized; it must be small enough to avoid diffuse reflection from the surface and high enough to provide superhydrophobicity. Although some groups have prepared surfaces combining the superhydrophobicity and high light transmission,31−39 there are still several drawbacks that hinder the practical outdoor applications of such multifunctional coatings. First of all, mechanical and thermal durability, which are essential for outdoor applications, are not sufficient or even not investigated for most of the coatings. Also, among such multifunctional coatings, highly water repellent ones generally exhibit antireflection property in a narrow range of wavelengths, or they can even diminish the light transmission of their substrates especially at low Received: October 23, 2012 Accepted: January 2, 2013 Published: January 2, 2013 853
dx.doi.org/10.1021/am3024417 | ACS Appl. Mater. Interfaces 2013, 5, 853−860
ACS Applied Materials & Interfaces
Research Article
added to the solution dropwise under gentle stirring. The reaction mixture was further stirred for 30 min and left for complete hydrolysis for 24 h at room conditions. Condensation was started by dropwise addition of 0.42 mL of ammonium hydroxide (25%) and 0.19 mL of water mixture under gentle stirring, and the reaction mixture was further stirred for 15 min. Finally, the solution was poured into a polystyrene vial and left for gelation at 25 °C. Gels were formed typically within one day, and they were aged for two days to strengthen the porous structure. After aging, 12 mL of methanol was added to the gels, and the resulting mixture was sonicated for 45 s at 20 W using an ultrasonic liquid homogenizer in order to obtain ormosil colloids.26,41 The colloids were spin coated on clean glass surfaces at 2000 rpm and dried at room temperature overnight. To increase the hydrophobicity and integrity of the coatings, they were heated to 450 °C for 1 h. The colloids and corresponding coatings were named with the number in the abbreviation indicating the TMOS volume fraction in the corresponding colloid, such that TM 7.5 indicates the coating produced using the colloid containing 7.5% TMOS and 92.5% MTMS. Preparation of Nonporous Ormosil Coatings. Nonporous ormosil coatings (NPF) were prepared by modifying the previous methods.42,43 In a standard procedure, 4.5 mL of MTMS (for NPF 1) or 3 mL of MTMS and 1.5 mL of TEOS (for NPF 2) were dissolved in 4.5 mL of ethanol and 0.4 mL of water, and 10 μL of 0.1 M HCl were added. The reaction mixture was stirred (250 rpm) at 60 °C for 90 min. Then, 0.8 mL of 0.1 M HCl and 0.7 mL of water were added, and the reaction mixture was further stirred for 15 min at room temperature. Then, the solution was aged at 50 °C for 15 min and diluted with an appropriate amount of ethanol. For example, to prepare 80 nm thick films, we added 14 mL of ethanol to the 2 mL of aged solution. Finally, 200 μL portions of diluted sol were coated on clean glass surfaces with spin coating at 2000 rpm. Numerical Simulations. To optimize the omnidirectional broad band antireflection property of the three layer coating, we performed a set of simulations based on the finite difference time domain (FDTD) method. We used plane wave source and Bloch boundary conditions to minimize simulation time. Transmitted light collected by frequencydomain power monitor. To plot transmission as a function of wavelength of light and incidence angle, we used 94 wavelength and 61 angle points in the 340−2200 nm wavelength and 0−60° ranges. Optimum refractive indices of bottom layer, middle layer, and top layer were determined as 1.4, 1.19, and 1.1, respectively, and corresponding thicknesses were determined as top, middle, and bottom layers are 240 nm, 140 nm, and 80 nm. Bulk glass assumed to have very large thickness and its refractive index is taken as 1.51. Detailed information about simulation software was given in ref 44. Preparation of Three Layer Coating. We prepared three layer coating by successive spin coating of ormosil solutions at 2000 rpm. First, the nonporous ormosil layer (NPF 2) was spin coated as described above and dried at room temperature overnight. To the top of this layer, 140 nm TM 60 was coated. To give the desired thickness, 4 mL of TM 60 colloid was diluted using 3 mL of methanol. This layer was dried at 225 °C for 1 h. Then, ∼240 nm TM 7.5 layer is coated to the top of the TM 60 layer. To give the desired thickness 2 mL of TM 7.5 colloid was diluted using 1 mL of methanol. Finally, coating was cured at 450 °C for 1 h. Characterization. The structures of the ormosil colloids were investigated with a transmission electron microscope (TEM, Tecnai G2 F30, FEI) operated at 200 kV. The samples were prepared on a holey carbon coated copper grid by placing a drop of the colloidal suspension used for coatings. SEM (E-SEM, Quanta 200F, FEI) was used to observe the structure of coatings at low vacuum condition or at high vacuum condition after coating the samples with 5 nm of gold− palladium or platinum. AFM (XE-100E, PSIA) was used in noncontact mode to determine the surface morphology and roughness of the coatings. Root mean square (RMS) roughness values of the coatings were calculated from three separate AFM images which were obtained from 10 × 10 μm2 regions. An Ellipsometer (V-Vase, J. A. Woollam) was used to measure the refractive index and thickness and to collect specular reflection spectra at oblique incidence angles of the coatings.
wavelengths (
